In one aspect the invention relates to a method and apparatus for detecting defects in a polymeric sheet or film. According to another aspect, the invention relates to a method and an apparatus for detecting defects in an oriented film.
In the past, defects in transparent and translucent polymeric sheets and films (hereinafter jointly referred to as sheets), have been detected by a visual inspection of the sheet. This visual inspection is a rather subjective test based on the laboratory technician's perceptions and, consequently, is prone to errors. Moreover, this visual inspection is mentally and physically fatiguing to the inspector, rendering it difficult to carry out continuous inspection operations. Therefore, there is a need to remove at least one source of human error from the detecting process by doing a computer aided count of defects.
In transparent and translucent polymeric sheets, typically polymer films, gel defects show up as thick spots in the sheets which can vary greatly in size. Most often they will appear transparent but a microscopic examination will indicate that they contain a gel or polymer particle at the core of the defect. Typically, these gel, or fisheye, defects will be comprised of small bits of plastic that were not completely melted before the formation of the sheet from the plastic. Because of their transparent or translucent nature, the gel defect can act to transmit light, thus, making them difficult to detect by prior detection methods which relied on the transmission of light through the sheet.